Is carbon dioxide in soda bad for you

We’re often warned to go easy on sparkling water, as it may be detrimental to our gut, bones and teeth. But is there any truth in this?

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We all know by now that drinking sweet, fizzy drinks all day isn’t a good idea. The combination of a high sugar content coupled with acidity caused by the carbonisation that makes it fizzy, isn’t good. Anyone who has tried leaving coins in a glass of cola overnight knows that they come up shiny and clean. The reason is that phosphoric acid in the drink removes the oxide coating that has built up on the coin. So one alternative is to drink water. “Still or sparkling?” they say to you in restaurants. If you’re not brave enough to say “tap” then sparkling can seem like a nice change.

The chances are though, that if you’re in a group at least one person will say sparkling water is bad for you, but is there any evidence for that claim?

Let’s start with the stomach. Fizzy water is made by adding carbon dioxide under pressure. The result is that water contains the weak acid, carbonic acid. If you gulp it down it can of course give you hiccups or indigestion. But what if you drink it at a more measured pace? Is there any truth in the idea that it harms your stomach?

Quite the reverse, it appears. In a small but double-blinded randomised trial, patients with frequent dyspepsia or constipation were assigned to drink either still or sparkling water for 15 days. Then they were given a series of tests. Both conditions improved in the people drinking sparkling water and showed no improvement in those drinking tap water.

If you drink a lot of sparkling water you might find you feel bloated, but researchers in Japan have found that this side-effect could be put to good use. They had a group of women fast overnight and then slowly drink either still or sparkling water. They found that 900ml of gas was released from just 250ml of water, so not surprisingly the women’s stomachs distended slightly and the had the perception of feeling full, even though they hadn’t eaten. They didn’t feel uncomfortable and so fizzy water has been suggested as a way of avoiding overeating, because it makes you feel fuller.

Bone problems?

And you might have heard people deliberately letting fizzy drinks go flat and then drinking them if they’re dehydrated after a stomach upset or vomiting or even a hangover. But a review of this practice in children with acute gastroenteritis found there’s little evidence that it works and that compared with rehydration powders – specifically constituted to contain replacement salts and sugars in the right proportions – such drinks contain far lower levels of sodium and potassium than you’d find in rehydration drinks. So it’s better to stick to the real thing.

But if sparkling water doesn’t damage your stomach, how about your bones?  Does it weaken them? Again, the evidence so far suggests not. A small, Canadian study published in 2001 found that teenagers who drank lots of fizzy drinks (not sparkling water) had less calcium in their bones, but they couldn’t tell whether this was a problem with the drinks themselves or that it was because people who drank them might favour them instead of milk.

The Framingham Heart study began in 1948 and followed a group of people over many years to discover more about the risk factors for heart disease. Now some of their offspring are taking part in the Framingham Osteoporosis Study which involves extensive testing every four years by researchers from Tufts University in Boston. In 2006, the team examined the relationship between bone density and fizzy drinks. They looked in detail at the different types of drink consumed by more 2,500 taking part in the study.

Some people have warned that any sparkling drink - even water - can be bad for us (Credit: Getty Images)

They found that the women (but not men) who drank cola-flavoured fizzy drinks three times a week had hip bones with a lower average bone mineral density. Other carbonated drinks made no difference. The authors hypothesise that the effect is probably down to caffeine and to the actions of phosphoric acid (not found in sparkling water) that are not yet well understood. It’s possible that it might somehow block calcium absorption – but no one yet knows how. Ten years later there is still disagreement over how diet affects bone health.

So as far as bones and stomachs go, so far drinking sparkling water seems to be fine. But how about teeth? Surely any acid, even a weak one, is going to erode the enamel on our teeth? Maybe not. Very little research has been done on sparkling water in particular, but much more has been done on other fizzy drinks. Barry Owens from the University of Tennessee College of Dentistry, Memphis, USA conducted a study back in 2007 comparing different fizzy drinks. In his study, cola-based drinks came out as most acidic, followed by diet-based cola drinks, followed by coffee.

Cumulative effect

He argues that it’s not just the initial pH of a drink that matters, but how strongly the drink retains that acidity in the presence of other substances, because in a real-life mouth saliva is present, as well as other foods which might affect the levels of acidity. This is known as the buffering capacity. A review of different drinks puts them in the following order for their buffering capacity. Non fruit-based carbonated drinks such as cola came out as the most acidic (with diet versions doing slightly better), followed by fruit-based fizzy drinks, fruit juice and then coffee. In other words, some fizzy drinks can damage the hardness of the enamel.

By taking slices of enamel and immersing them in different soft drinks for six, 24 and 48 hours, Poonam Jain at Southern Illinois University School of Dental Medicine demonstrated that the enamel does begin to erode. Some argue that this isn’t very like real life because we don’t keep a drink in our mouth for that length of time. But over the course of many years, even a few seconds each slurp adds up.

The eroding effects of sweet fizzy drinks add up over time (Credit: Getty Images)

A case study published in 2009 of a 25-year-old bank worker whose front teeth wore out after four years of drinking half a litre of cola a day, followed by three years where he upped that to a litre-and-a-half each day and added in some fruit juice, is enough to frighten anyone. But it also depends on how you drink it. This man was described as “holding the drink in the mouth for several seconds and tasting before swallowing”. In Sweden researchers compared short-sipping, long-sipping, gulping, nipping (whatever that might be) and sucking. They found that the longer a drink stayed in the mouth, the more noticeable the drop in pH in that person’s mouth. In other words, the more acidic the mouth becomes. But if you drink through a straw the drink goes straight to the back of your mouth and there’s less opportunity for damage.

But what about sparkling mineral water? At the University of Birmingham, Catriona Brown put extracted human teeth without signs of erosion into jars for 30 minutes with different kinds of flavoured sparkling water to see what happened. The teeth had been coated in varnish, apart from a half-a-centimetre-diameter test area which was left unvarnished. They found the effect of the drinks on the teeth was the same and sometimes greater than the effect of orange juice, a drink which is already known to soften tooth enamel. Lemon and lime, and grapefruit were the most acidic flavours, probably because they use citric acid to give the nice taste.

Sparkling water turns out to be only 1% as acidic as sugary sodas, research suggests (Credit: Getty Images)

So flavoured mineral waters shouldn’t be considered as harmless as water, but how about sparkling water with no added flavours?  Studies on this are few and far between. But in 2001, the Birmingham team examined seven different brands of mineral water, again pouring them over extracted teeth to see what happened. They found sparkling waters had a pH of between 5 and 6 (so not as acidic as some cola drinks which can be as high as 2.5), compared with still water which was neutral at 7. In other words, they are a weak acid, as suspected. But when it came to the erosive potential of that weak acid on the teeth, the effect was 100-times less than that of some other kinds of fizzy drinks. Of course the mouth itself is a different environment from a jar, but so far the evidence for harm doesn’t seem to be very strong.

So if you want a change from plain old water, then although it’s mildly acidic, so far there isn’t strong evidence to suggest that it’s harmful to your bones, your stomach or your teeth. But if you want to play safe and keep it away from your teeth, when you answer the question “still or sparkling”, perhaps you should also ask for a straw. 

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